Ignore the entire 'team fortress 2' directory.

In the event that configuration files/scripts are being edited inside
the actual Steam directory in makes sense to ignore the contents of the
game's subdirectory.

What is beautiful about this is that it means that the project can be
cloned directly (with a bit of copying and pasting so Git doesn't
complain) into '$(STEAM_DIR)\steamapps\$(USERNAME)\team fortress 2' and
be maintained from there.